#! /usr/bin/gawk -f

BEGIN { 
  k = 0; 
  nk = length(key); 
  alpha = "abcdefghijklmnopqrstuvwxyz";
  if (decode == 1) { dir = 25; } else { dir = 1; }
}

{ ns = length($0);
  for (i=1; i<=ns; i++)
    { cp = substr($0,i,1); ip = index(alpha, cp) - 1;
      if (ip >= 0) 
        { ck = substr(key,k+1,1); ik = index(alpha, ck) - 1;
          ip = (ip + dir*ik) % 26;
          cp = substr(alpha, ip+1, 1);
          k = (k + 1) % nk;
        }
      printf "%s", cp;
    }
  printf "\n";
}